1. Field of the Invention
This invention relates to electrophoretic devices such as electrophoretic displays which contain electrophoretic dispersion liquids containing electrophoretic particles dispersed in a liquid phase dispersion medium, wherein the electrophoretic dispersion liquid is arranged between electrodes that are arranged opposite to each other and are electrified to attract the particles in different directions respectively. In addition, this invention also relates to methods of manufacturing the electrophoretic devices.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. Hei 9-185087 discloses the general structure of a conventional electrophoretic display. FIG. 8A shows a structure of the disclosed electrophoretic display, wherein an electrode 3 is arranged on a substrate 1 while a transparent electrode 4 is arranged below a transparent substrate 2. The space between these electrodes 3 and 4 is filled with electrophoretic dispersion liquid 50. In addition, spacers 7 are provided to prevent bleeding of the electrophoretic dispersion liquid 50 from the electrophoretic display. Incidentally, FIG. 8A shows only a selected section of the electrophoretic display for one pixel.
The electrophoretic dispersion liquid 50 contain electrophoretic particles 5 that are dispersed in liquid phase dispersion medium 6. Different colors are applied to the electrophoretic particles 5 and the liquid phase dispersion medium 6 respectively.
The electrophoretic display is connected to two voltage sources 9a and 9b by way of a switch 8. The voltage sources 9a and 9b apply voltages between the electrodes 3 and 4 with opposite polarities. That is, the electrode 4 is directly connected with the negative terminal of the voltage source 9a and the positive terminal of the voltage source 9b, whereas the electrode 3 is connected with either the positive terminal of the voltage source 9a or the negative terminal of the voltage source 9b by way of the switch 8. The aforementioned connections allow alternation of the polarity of the voltage by switching the switch 8. By simply changing the direction of the voltage applied between the electrodes 3 and 4, the electrophoretic dispersion liquid 50 is adequately subjected to polarization to enable the desired images to be displayed by the electrophoretic display. When selectively applying the voltage of the voltage source 9a between the electrodes 3 and 4 as shown in FIG. 8B, the electrophoretic particles 5 move towards the transparent electrode 4, which is observed by a human observer. In this state, the human observer views the color of the electrophoretic particles 5 with his/her eyes. When selectively applying the voltage of the voltage source 9b between the electrodes 3 and 4 as shown in FIG. 8C, the electrophoretic particles 5 move towards the electrode 3, which is arranged opposite to the transparent electrode 4 being observed by the human observer. In this state, the human observer views the color of the liquid phase dispersion medium 6.
The aforementioned structure of the electrophoretic display shown in FIG. 8A allows the human observer to view two colors, which can be displayed on the screen by switching the direction of the voltage applied between the electrodes 3 and 4. Therefore, the overall configuration of the electrophoretic display can be realized by providing the aforementioned structure for each pixel on the screen.
It is possible to produce an electronic paper by forming the aforementioned electrophoretic display on the flexible board or the like. Such electronic paper requires the displayed images to be retained over time. That is, once some display contents are written and are displayed on the electronic paper, it is necessary to retain the display contents even though a voltage is no longer applied.
Japanese Examined Patent Publication No. Sho 50-15115 teaches a technique for retaining the display contents. Specifically, it teaches the use of a specific dispersion medium that is solidified at normal temperatures and that can be softened by heat or a solvent. The aforementioned technique requires complicated steps in writing images, such as heating steps.
Japanese Unexamined Utility-Model Publication No. Hei 2-51325 discloses another technique that uses liquid crystal for the dispersion medium. However, this technique is disadvantageous because only a limited range of materials can be used for the dispersion medium.
U.S. Pat. No. 5,961,804 discloses an electrophoretic display having the specific structure in which charged elements having the polarity opposite to that of the particles are restrained inside of microcapsules containing dispersion liquids. However, it is very difficult to carry out the charge restraining method, which complicates the manufacturing processes. In addition, it is disadvantageous because only a limited range of materials can be used for the microcapsules.
Japanese Unexamined Patent Publication No. Hei 3-213827 discloses a technique for refreshing displayed images by periodically applying a drive voltage to the electrophoretic display. However, it is complicated in control and is disadvantageous because of the large consumption of electricity. For this reason, it is very difficult to apply the aforementioned technique to the electronic paper.
It is an object of the invention to provide an electrophoretic device that has remarkably improved display characteristics for retaining images on the screen when producing an electronic paper.
It is another object of the invention to provide a method for manufacturing the electrophoretic display with simple processes and practically without limitations in selecting the materials for use in the manufacture.
This invention provides unique and easy-to-manufacture structures for electrophoretic devices such as electrophoretic displays, in which an electrophoretic dispersion liquid containing electrophoretic particles dispersed in the liquid phase dispersion medium is contained in a space between electrodes, at least one of which is composed of transparent materials. This invention is characterized by arranging ferroelectric substance layers between the electrodes, which are arranged opposite to each other. The provision of the ferroelectric substance layers, which are arranged between the electrodes, allows the polarized state of the electrophoretic dispersion liquid to be maintained for a long time. Thus, it is possible to improve the image retaining capability of the electrophoretic device.
In the first aspect of the invention, the ferroelectric substance layers are connected in series within the space formed between the electrodes. Therefore, it is possible to enlarge the overall effective area of the ferroelectric substance layers, which makes it effectively maintain the polarized state of the electrophoretic dispersion liquid. In addition, it is possible to simplify the structure of the electrophoretic device, which can be manufactured with ease.
In the second aspect of the invention, ferroelectric substance layers are arranged in parallel between the electrodes. In this case, it is possible to reduce the drive voltage applied between the electrodes because of the parallel arrangement of the ferroelectric substance layers between the electrodes.
In the third aspect of the invention, there is further provided a switch enabling repetitive and reversible switching between the electrophoretic dispersion liquid and the ferroelectric substance layers to be electrically connected in series or in parallel. When the ferroelectric substance layers and the electrophoretic dispersion liquid are electrically connected in parallel, it is possible to reduce the drive voltage, which is applied between the electrodes. During the non-drive mode, the ferroelectric substance layers and the electrophoretic dispersion liquid are electrically connected in series, which makes it possible to effectively retain the polarized state of the electrophoretic dispersion liquid.
Because the ferroelectric substance layers are composed of organic polymer ferroelectric substances, it is possible to form the ferroelectric substance layers by prescribed printing and painting methods using an organic solvent solution, for example. This provides an easy way to manufacture the electrophoretic device.
At least one of the electrodes is partitioned into a plurality of sections in relation to a plurality of pixels on the screen. In addition, the ferroelectric substance layers can be also partitioned into a plurality of sections in relation to the pixels respectively. These sections of the ferroelectric substance layers are not necessarily provided for all of the pixels on the screen. Hence, it is possible to provide them for desired locations of the pixels respectively. This enables different contents to be displayed with respect to the pixels respectively. Therefore, it is possible to display the desired images on the screen.
The ferroelectric substance layer is formed by a prescribed method using an organic solvent solution for use in organic polymer ferroelectric substances and the like. As the prescribed method, it is possible to employ the inkjet printing method, screen process printing method, spin-coating method, roll-coating method, etc. These methods do not require the special equipment such as a vacuum chamber. Hence, it is possible to manufacture the electrophoretic device at a relatively low cost.
A first manufacturing method is to form the layer of the electrophoretic dispersion liquid after formation of the ferroelectric substance layers in accordance with a prescribed method. Thus, it is possible to provide a parallel arrangement of the ferroelectric substance layers, which create spaces for arranging microcapsules between the electrodes.
A second manufacturing method is to form the ferroelectric substance layers after the formation of the layer of the electrophoretic dispersion liquid in accordance with a prescribed method. Thus, it is possible to provide a serial arrangement of the ferroelectric substance layers within the space formed between the electrodes.